1. Field of the Art
The disclosure herein relates to the field of digital media compression, and more particularly, to compressing reference frames in video.
2. Description of the Related Art
Conventional digital media technology is based mainly around the MPEG-2 (Motion Picture Expert Group) standard or the MPEG-4 Part 2 standard. Significant advancements have been made to media compression technology with the more recent development of H.264, also known as MPEG-4 Part 10 or MPEG-4 AVC. The H.264 format is able to code media in a more highly compressed form which reduces memory requirements and improves network performance. However, media consumers and producers may be reluctant to transition to the new standard because of the costs associated with replacing the existing MPEG compatible media and media players. The transition can be eased by making the new technology compatible with existing technology. In some instances, compatibility is enhanced by designing systems that can perform conversions between different compressed media standards.
Transcoders are devices configured to convert media from a first compressed format to a second compressed format. A transcoder consists of a decoder to decode a compressed input media stream of a first compressed format (such as MPEG-2) into an intermediate format, a processing unit to process the media stream in its intermediate form, and an encoder to encode the processed media stream into a compressed output stream of a second compressed format (such as H.264). The processing unit may perform various operations on the media stream such as video scaling.
Traditional decoders, encoders, and transcoders rely on uncompressed “reference frames” in the encoding and decoding processes. In conventional video compression, movement from frame to frame is encoded by a motion vector and an error term. The motion vector describes the relative position of a macroblock in the current frame to a similar macroblock-sized region in a reference frame. The error term gives the difference between the current macroblock and the referenced macroblock-sized region. Since predicted frames rely on the reference frames as a basis for encoding and decoding the image, any errors in the reference frames can propagate to the other frames. This problem, known as drift, can create artifacts in the video stream. In a conventional decoder, the reference frames are typically stored uncompressed in order to preserve video quality.
However, uncompressed media frames, especially video, require bulky processing hardware to handle the high bandwidth for data transfers. Uncompressed video also requires significant amounts of storage. This increases system cost. In addition, the performance of the transcoder suffers since the uncompressed frames take up significantly more bandwidth than compressed frames.